In accordance with the development of 21C information and communication technologies and a desire for a personal portable communication device, a micromachining process enabling an information and communication device having a small size, a light weight, and a thin thickness, and capable of being easily used, a high-performance electric and electronic material capable of manufacturing an ultra high density integrated circuit, and a novel information and communication material capable of implementing a new concept display have been required. Among them, since an organic thin film transistor (OTFT) may be used as an important component of display drivers of portable computers, organic electro luminescence devices, smart cards, electric tags, pagers, mobile phones, or the like, and plastic circuit parts of memory devices such as automated teller machines, identification tags, or the like, etc., the organic thin film transistor (OTFT) has been the subject of various studies.
The organic thin film transistor using an organic semiconductor has advantages such as a simple manufacturing process and low production cost as compared to organic thin film transistors using amorphous silicon and polysilicon up to now, excellent compatibility with plastic boards for implementing flexible displays, or the like. Therefore, recently, various researches into the organic thin film transistor using an organic semiconductor have been conducted. Particularly, since a thin film may be easily formed by a solution process in a case of using a polymer organic semiconductor, manufacturing cost may be decreased as compared to a small molecular organic semiconductor compound.
As a representative semiconductor compound for a polymer based organic thin film transistor developed up to now, there are poly(3-hexylthiophene) (P3HT) and poly(9,9-dioctylfluorene-co-bithiophene) (F8T2). The OTFT has various performances, but among them, important evaluation barometers are charge mobility and an on/off ratio. Among them, the most important evaluation barometer is the charge mobility. The charge mobility is different depending on the kind of semiconductor material, a formation method (structure and morphology) of a thin film, a driving voltage, or the like.
FIG. 1 is a cross-sectional view of a general organic thin film transistor composed of a substrate/gate/insulating layer/electrode layer (source, drain)/organic semiconductor layer. Referring to FIG. 1, a gate electrode is formed on the substrate. The insulating layer is formed on the gate electrode, and the organic semiconductor layer and source and drain electrodes are sequentially formed thereon. The driving principle of the organic thin film transistor having the above-mentioned structure will be described below with an example of a p-type semiconductor. First, in the case of applying a voltage between a source and a drain to flow a current, a current in proportion to the voltage flows at a low voltage. Here, when a positive voltage is applied to a gate, all holes, which are positive charges, are pushed toward an upper portion of the semiconductor layer by an electric field due to the applied voltage. Therefore, a depletion layer in which there is no conductive charge is formed in a portion adjacent to the insulating layer, and even though a voltage is applied between the source and the drain, since conductive charge carriers are decreased in this situation, a low amount of current will flow. On the contrary, when a negative voltage is applied to the gate, an accumulation layer in which positive charges are induced is formed in a portion adjacent to the insulating layer by an effect of the electric field due to the applied voltage. In this case, since a large amount of conductive charge carrier is present between the source and the drain, a larger amount of current may flow. Therefore, a current flowing between the source and the drain may be controlled by alternatively applying the positive voltage and the negative voltage to the gate in a state in which the voltage is applied between the source and the drain.
Examples of components used in the organic thin film transistor configured as described above include electrodes (source and drain), a substrate and a gate electrode, which are required to have high thermal stability, an insulator required to have high insulation properties and dielectric constant, a semiconductor smoothly moving charges, and the like. Among them, a core material having many problems to overcome is an organic semiconductor. The organic semiconductor may be divided into a small molecular organic semiconductor and a polymer organic semiconductor depending on a molecular weight thereof, and may be classified into an n-type organic semiconductor and a p-type organic semiconductor depending on whether or not the organic semiconductor transports electrons or holes. Generally, in the case of using a small molecular organic semiconductor at the time of forming an organic semiconductor layer, since the small molecular organic semiconductor may be easily purified and thus impurities may be almost removed, charge transfer properties may be excellent. However, since it is impossible to perform spin-coating or printing, a thin film should be manufactured by a vacuum deposition method, such that a manufacturing process is complicated and expensive as compared to the polymer organic semiconductor. In the case of the polymer organic semiconductor, it is difficult to purify the polymer organic semiconductor with a high purity, but thermal resistance is excellent, and it is possible to perform spin-coating and printing, such that the polymer organic semiconductor has advantages in view of a manufacturing process, cost, and mass production.
In order to develop organic semiconductor materials, many studies have been conducted up to now, but the development of polymer based semiconductor materials is still far below the development of small molecule based semiconductor materials. Therefore, in order to develop an electronic device using an organic thin film transistor which is flexible and is cheaply manufactured, the development of a polymer based semiconductor material has been urgently demanded. Generally, it is known that charge mobility of a polymer is lower than that of a small molecule, but the polymer may be a material capable of sufficiently overcoming this disadvantage in view of a manufacturing process or cost.
A polymer in which an S containing heteroaromatic ring is directly bound to a diketopyrrolopyrrole group has been disclosed in Korean Patent Laid-Open Publication Nos. 2011-0091711 and 2009-0024832. However, since sufficient expansion of π-electron is not still exhibited, the development of a polymer semiconductor material exhibiting sufficient π-electron stacking has been required.